An exact ignition timing is important to the performance of the engine. A spark plug in the cylinder usually emits spark several degrees before the piston reaches the upmost position (or Top Dead Center). This job is done by the coordination of distributor axle and camshaft. However once the cam surface or the braker point is worn out the ignition timing becomes inaccurate. Under this circumstance, the explosion in the cylinders will be either too early or too late which results in poor engine performance. The ignition timing of a gasoline engine thus needs regular adjustment to assure its exact timing.
Timing is usually done by a timing gun which emits lights to timing marks (71) commonly found on the vibration damper (7) of crank shaft (6). The power of timing light is induced from the current to spark plug by introducing a induction clamp on the distribution wire to the plug. The induced current is amplified to actuate the xenon bulb to emit flashing light simultaneously with the firing of a spark plug. The emitted light shall be aiming at the marks on the damper. Because the emitting of the timing light is synchronized with the engine explosion, we will be able to see the timing marks in a motionless-like situation. The ignition timing is then indicated by the relative position between the marks and a pointer (81) on the timing gear cover (8).
Ignition timing, however, is a variant of engine speed. When engine is working at a higher speed, it has very short combustion time. To make sure of a more complete combustion, the ignition timing has to be advanced a little bit earlier. Otherwise the piston will reach upmost position and start to be pushed downward before the fuel is completely ignited. For this reason, the timing work has to be done under certain engine speed (RPM) according to the specification of the automobile manufacturer. The instrument for testing engine speed is a so called "Tachometer".